1. Field of the Invention
This invention relates to a protective device for an integrated circuit and the manufacturing method thereof, more particularly to a protective device and the manufacturing method thereof which prevents large current flows from a driving circuit stage to a driven circuit stage of the integrated circuit.
2. Description of the Related Art
Presently, metal-oxide semiconductor devices are widely used in integrated circuits. In accordance with currently available semiconductor manufacturing technology, the thickness of a gate oxide of a metal-oxide-semiconductor field effect transistor (MOSFET) is only 120.ANG. to 250.ANG.. Therefore, breakdown of the gate oxide of the MOSFET easily occurs when there is no protective device for preventing undesired large voltage across the MOSFET, thereby resulting in damage to the integrated circuit.
FIG. 1 shows a conventional protective device 1 for an integrated circuit with a driving circuit stage (not shown) and a driven circuit stage 12. The conventional protective device 1 has first and second diodes (13 and 14). The first diode 13 has a cathode connected electrically to an output terminal 11 of the driving circuit stage via a P+-type diffusion resistor (RS) and an anode connected electrically to a ground reference voltage (VSS). The second diode 14 has a cathode connected electrically to a voltage source (VDD) of the driven circuit stage 12 and an anode connected electrically to the output terminal 11 of the driving circuit stage via the diffusion resistor (RS) and to the cathode of the first diode 13. The cathode of the first diode 13 and the anode of the second diode 14 are further connected electrically to the gate terminals of the MOSFET (M1,M2) of the driven circuit stage 12.
When a positive voltage slightly larger than the sum of the voltage of the voltage source (VDD) and 0.7 v is present at the output terminal 11 of the driving circuit stage, the second diode 14 conducts, thereby preventing large current flows from the driving circuit stage to the driven circuit stage 12. On the other hand, when a negative voltage slightly smaller than the sum of the ground reference voltage (VSS) and -0.7 v is present at the output terminal 11 of the driving circuit stage, the first diode 13 conducts, thereby preventing large current flows from the driving circuit stage to the driven circuit stage 12 in the reverse direction. It should be noted that the diffusion resistor (RS) has a large potential difference thereacross when a large current flows therethrough, thereby reducing the voltage at the gate terminals of the MOSFETs (M1,M2) so as to protect the latter. Furthermore, the diffusion resistor (RS) reduces the amount of current flow so as to reduce the joule heat, thereby preventing damage to the integrated circuit due to high temperature.
However, when a voltage much larger than the sum of the voltage of the voltage source (VDD) and 0.7 v or much smaller than the sum of the voltage of the ground reference voltage (VSS) and -0.7 v is present at the output terminal 11, a large current flows from the driving circuit stage to the driven circuit stage 12, thereby resulting in overload of the driving circuit stage. Furthermore, if the current exceeds the trigger current, a latch-up effect occurs. Therefore, the voltage present at the output terminal 11 of the driving circuit stage should be limited within a range of the sum of the voltage of the voltage source (VDD) and 0.7 v and the sum of the voltage of the ground reference voltage (VSS) and -0.7 v to ensure proper operation of the protective device.
Referring to FIG. 2, another conventional protective device for an integrated circuit is shown. An n-type thick field oxide MOSFET (M3) has its gate and drain terminals connected electrically to the output terminal 21 of the driving circuit stage. A first resistor (R1) has a first terminal connected electrically to the drain terminal of the transistor (M3) and a second terminal. A second resistor (R2), which is an N+-type diffusion resistor, has a first terminal connected electrically to the second terminal of the first resistor (R1) and a second terminal connected electrically to the gate terminals of the transistors (M1,M2) of the driven circuit stage.
When a large positive voltage is present at the output terminal 21 of the driving circuit stage, the N-channel of the transistor (M3) conducts and forms a discharge path. At this time, the amount of current flow is relatively small. The drain voltage of the transistor (M3) increases continuously until avalanche breakdown of an N+P- diode formed between the drain terminal and the substrate of the transistor (M3) occurs due to reverse bias. At this stage, a large amount of substrate current flows, and a voltage difference is formed between the source terminal and the substrate of the transistor (M3). The voltage difference triggers the parasitic bipolar transistor so as to turn-on the transistor (M3) in order to permit the dynamic resistance and the substrate resistance of the diode formed by the drain and substrate terminals to be reduced from a number of K.OMEGA. to a number of .OMEGA.. Furthermore, since the diode generates a snapback effect due to the avalanche breakdown caused by the reverse bias, the avalanche breakdown voltage is reduced 3 to 10 volts. When the voltage present at the output terminal 21 of the driving circuit stage is lower than the voltage difference between the ground reference voltage (VSS) and the cut-in voltage of the diode, an N+P- diode formed by the drain terminal and the substrate of the transistor (M3) conducts to form a discharge path due to the forward bias.
From the foregoing, it is noted that the voltage present at the output terminal 21 of the driving circuit stage cannot be higher than the reverse breakdown voltage of the drain terminal of the transistor (M3), or cannot be lower than the voltage difference between the ground reference voltage and the cut-in voltage of the diode. Otherwise, a large current flows so as to result in overload of the driving circuit stage and in a latch-up effect thereof. Therefore, in practice, if the voltage output from the driving circuit stage has a relatively absolute value, a voltage level converter must be added to overcome the aforementioned problems.